An important distinction: "Not guilty by reason of insanity" and "diminished
capacity"

Although a defense known as "diminished capacity" bears some
resemblance to the "reason of insanity" defense (in that both examine the
mental competence of the defendant), there are important differences. The
most fundamental of these is that, while "reason of insanity" is a full
defense to a crime -- that is, pleading "reason of insanity" is the equivalent
of pleading "not guilty" -- "diminished capacity" is merely pleading to
a lesser crime.

One of the most famous recent uses of the insanity defense came in United
States v. Hinckley, concerning the assassination attempt against
then-President Ronald Reagan.

The history of "not guilty by reason of insanity"

The insanity defense reflects a compromise on the part of society and the
law. On the one hand, society believes that criminals should be punished
for their crimes; on the other hand, society believs that people who are
ill should receive treatment for their illness. The insanity defense is
the compromise: basically, it reflects society's belief that the law should
not punish defendants who are mentally incapable of controlling their conduct.

In the 18th century, the legal standards for the insanity defense were
varied. Some courts looked to whether the defendant could distinguish between
good and evil, while others asked whether the defendant "did not know what
he did." By the 19th century, it was generally accepted that insanity was
a question of fact, which was left to the jury to decide.

The McNaughton rule -- not knowing right from wrong

The first famous legal test for insanity came in 1843, in the McNaughton
case. Englishman Daniel McNaughton shot and killed the secretary of the
British Prime Minister, believing that the Prime Minister was conspiring
against him. The court acquitted McNaughton "by reason of insanity," and
he was placed in a mental institution for the rest of his life. However,
the case caused a public uproar, and Queen Victoria ordered the court to
develop a stricter test for insanity.

The "McNaughton rule" was a standard to be applied by the jury, after
hearing medical testimony from prosecution and defense experts. The rule
created a presumption of sanity, unless the defense proved "at the time
of committing the act, the accused was laboring under such a defect of
reason, from disease of the mind, as not to know the nature and quality
of the act he was doing or, if he did know it, that he did not know what
he was doing was wrong."

The McNaughton rule became the standard for insanity in the United States
and the United Kingdom, and is still the standard for insanity in almost
half of the states.

The Durham rule -- "irresistible impulse"

Monte Durham was a 23-year-old who had been in and out of prison and
mental institutions since he was 17. He was convicted for housebreaking
in 1953, and his attorney appealed. Although the district court judge had
ruled that Durham's attorneys had failed to prove he didn't know the difference
between right and wrong, the federal appellate judge chose to use the case
to reform the McNaughton rule.

Citing leading psychiatrists and jurists of the day, the appellate judge
stated that the McNaughton rule was based on "an entirely obsolete
and misleading conception of the nature of insanity." He overturned Durham's
conviction and established a new rule. The Durham rule states "that
an accused is not criminally responsible if his unlawful act was the product
of mental disease or mental defect."

The Durham rule was eventually rejected by the federal courts,
because it cast too broad a net. Alcoholics, compulsive gamblers, and drug
addicts had successfully used the defense to defeat a wide variety of crimes.

The Model Penal Code: turning responsibility to the jury

In 1972, the American Law Institute, a panel of legal experts, developed
a new rule for insanity as part of the Model Penal Code. This rule says
that a defendant is not responsible for criminal conduct where (s)he, as
a result of mental disease or defect, did not possess "substantial capacity
either to appreciate the criminality of his conduct or to conform his conduct
to the requirements of the law." This new rule was based on the District
of Columbia Circuit's decision in the federal appellate case, United
States v. Brawner, 471
F.2d 969 (1972).

Obviously, this standard is very vague. It leaves a number of factors
up to the jury to determine, given the facts of a case and the testimony
of experts. About half the states have adopted the Model Penal Code rule
for insanity.

The Federal rule: Reagan gets into the act

In 1984, Congress passed, and President Ronald Reagan signed, the Comprehensive
Crime Control Act. The federal insanity defense now requires the defendant
to prove, by "clear and convincing evidence," that "at the time of the
commission of the acts constituting the offense, the defendant, as a result
of a severe mental disease or defect, was unable to appreciate the nature
and quality or the wrongfulness of his acts" (18
U.S.C. § 17). This is generally viewed as a return to the "knowing
right from wrong" standard. The Act also contained the Insanity Defense
Reform Act of 1984, 18
U.S.C. § 4241, which sets out sentencing and other provisions
for dealing with offenders who are or have been suffering from a mental
disease or defect.